With fans of the above-referenced type, of which many embodiments are known, one constant development objective is to maximize the attainable efficiency. Efficiency is understood in the narrower sense-only with reference to the fan wheel-to be the ratio of the flow rate of the fan to the power requirement of the fan shaft. For a given shaft power, the efficiency is determined from the conveyed gas volume flow and the total pressure increase produced by the fan. The product of these gives the flow rate, with total pressure being understood according to the so-called Bernoulli equation as the sum of static and dynamic pressure.
This development objective, wherein each percentage point of efficiency increase is important, has gained in relevance in connection with the new formulation of the so-called Ecodesign Directive or ErP Directive (Directive 2009/125/EG of the European Union), which comprises not only energy-using products, but generally energy-related products for the minimization of harmful environmental pollution. This applies both to fans operated as stand-alone devices and to fans that are integrated as components of a device or system. The ErP Directive thereby specifies minimum efficiencies, whereby here efficiency of the entire fan, that is, the unit consisting of the control electronics (if present), motor, and fan wheel is assessed as efficiency in the broader sense.
It is the object of the invention to create an axial fan of the kind described previously, by which improvements of operating performance with regard to efficiency can be attained in a less costly manner. The noise level should not be affected by this, but preferably would be improved.
This object is attained according to the invention by mounting a rotation-symmetric flow guiding element on the outlet side directly or indirectly on the stator and concentrically with respect to a stator hub. Said flow guiding element has an outer diameter that is greater than the outer diameter of the stator hub, greater than the outer diameter of the rotor, and smaller than the diameter of a perimeter around the blades. Such a flow guiding element provided according to the invention, which is mounted in the direction of flow behind the fan, acts advantageously as a blockage or deflector plate for the conveyed gas flow, and prevents a volume flow from being drawn from a region of turbulence behind the motor. Such turbulence zones, which are produced after a stream separates from a body around which it has been flowing, are also called “dead water zones” or “eddy flow regions” in the case where a liquid is the flow medium. There is no laminar flow there.
In addition, the flow guiding element provided according to the invention also ensures that a backflow of flow medium can only begin within a diameter range that is greater than the diameter of the hub, thus restricting backflow. This occurs, in turn, because the backflow must take place against centrifugal force, which increases proportionally with the distance to the fan axis. A backflow zone that arises in particular when using axial fans with high counterpressure can be kept comparatively low according to the invention. Because the flow through the fan separates only when the diameter is greater than the diameter of the hub, up to the time of its separation it can also reach a comparatively higher pressure than when the flow guiding element is not present. The total attainable pressure increase with the fan thereby rises, and the efficiency increases by a few percentage points in comparison with a fan without such a flow guiding element.
The flow guiding element provided according to the invention can be advantageously used together with a protective screen likewise mounted at the outlet side on the stator of the axial fan. At the same time, it can be configured as a separate component or can be integrated into the protective screen, that is to say, mounted in particular thereon or therein. A one-piece configuration with the protective screen is also possible in this regard.
The flow guiding element can be advantageously mounted as a plastic or metal part that may be clipped on, screwed in, riveted, or welded, that is, it can be friction-fitted, form-fitted and/or bonded, in particular in a manner that is less costly from the point of view of production and assembly as a plastic part that can be clipped on or can be configured as a weldable metal part with higher strength in a more robust design.
In a preferred embodiment, the flow guiding element can be configured in a conical shape, in particular with the basic shape of a truncated cone, wherein, when mounted, its jacket surface diverges in a direction facing away from the stator. This has the advantageous effect that the slanted position of the jacket surface causes the flow to be deflected outward and that the flow guiding element thus acquires a better guiding capacity.
Other advantageous embodiments of the invention are contained in the following description.